KEY POINTS
- This single-center retrospective study included 74 patients with macroscopic adenoid cystic carcinoma treated with curative-intent carbon-ion radiotherapy from 2016 to 2023. Prescriptions were 68.8 or 65.6 Gy(RBE) in 16 fractions, planned using the local effect model I.
- After a median follow-up of 44 months, 23 patients (31%) developed local recurrence. Baseline clinical characteristics and prescription distributions were comparable between the local-control and recurrence groups.
- At 68.8 Gy(RBE), conventional dose–volume metrics did not differ by outcome under either relative biological effectiveness model. At 65.6 Gy(RBE), significant differences in D50%, D95%, D98%, and D99% emerged only after recalculation with the modified microdosimetric kinetic model.
- Dose–linear energy transfer–volume analysis identified the strongest outcome discrimination at dose-averaged linear energy transfer values above approximately 40 keV/µm. High-dose, high-linear-energy-transfer coverage significantly stratified local relapse-free survival.
- Outcome-derived planning thresholds were 64.0 Gy(RBE) with 44 keV/µm covering at least 80% of the targetunder the local effect model I and 56.3 Gy(RBE) with 43 keV/µm covering at least 90% under the modified microdosimetric kinetic model.
CLINICAL TAKEAWAY
Carbon-ion target evaluation based on dose alone may miss biologically relevant undercoverage, particularly when plans are interpreted across different relative biological effectiveness models. Combined dose and linear energy transfer objectives could support more biologically informed optimization for adenoid cystic carcinoma, but these single-center retrospective thresholds require prospective and multicenter validation before routine adoption.
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